I flew with Easyjet last year and about a month before departure I had to inform the airline that I was carrying an air rifle.
This proved a straightforward procedure. However, at the check-in desk the people didn't seem well informed. I found it useful to have printed off my e-mail conversation with the Easyjet Customer Support.
The e-mail address to contact them is:
easyJet Customer Experience Team
In my first contact I quoted the flight number and that I was carrying an air rifle and then they asked me for further information.
Sorry if this is all old info but if anyone wants anymore please just ask